1. Field of the Invention
The present invention relates generally to a method and apparatus for removing toxic substances from waste marine products, and more particularly to a method and apparatus for removing toxic substances from waste marine products, which extracts the toxic substances such as heavy metal and arsenic included in the waste marine products such as cuttlefishes, shellfishes, shells (scallops in particular) and fishes into an acid solution and separates/collects the toxic substances.
2. Description of Related Art
Sometimes, the toxic substances such as heavy metal (e.g., silver, copper, cadmium and zinc), arsenic and phosphorus are concentrated in the internal organs, liver and kidney of fishes and cuttlefishes and the internal organs of shells. The concentration of the heavy metal in the marine products varies according to the kinds and organs of the marine products, the seasons and the habitats. The concentration of the heavy metal is high from the summer to the autumn when the temperature is high, and the heavy metal of 30-100 mg/dried solid 1 kg or more is included in the marine products. When the temperature is low, the heavy metal of several ten mg may be included in the marine products. Arsenic of several ten mg may also be included in some kinds and organs of the marine products.
Waste marine products including the toxic substances such as heavy metal and arsenic are buried in the ground, thrown into the sea and incinerated. If a large amount of waste marine products is discarded, they become toxic wastes that may pollute the environment. It is therefore preferable to discard the waste marine products after the toxic substances such as heavy metal and arsenic are removed from the waste marine products in order to prevent the pollution of the environment.
Since the waste marine products includes protein of high concentration, the waste marine products may be reused for animal food if the concentration of the heavy metal per 1 kg of dried solid is 5 mg or less and the concentration of arsenic per 1 kg of dried solid is 1 mg or less. The concentration of heavy metal and arsenic are allowed to be higher than feed if the waste marine products are reused for fertilizers.
Accordingly, Japanese Patent Provisional Publication Nos. 8-99001 and 9-47257 disclose the technique of extracting the heavy metal included in the waste marine products into a sulfuric acid solution, or the like and electrolytically removing the heavy metal included in the extracted solution. This technique has been developed around Hokkaido prefectural industrial laboratory.
In a conventional method of removing the heavy metal in the electrolysis, the heavy metal in the waste marine products is proved to be eliminated at a high removal rate of 99% or more, but the waste marine products must be processed for dozens of hours (Japanese Patent Provisional Publication No. 8-99001) and for about twenty-four hours (Japanese Patent Provisional Publication No. 9-47257) until the concentration of the heavy metal is reduced to an allowable value of 5 mg/dried solid 1 kg.
In addition, the deposit including the heavy metal is accumulated on the surfaces of the electrodes, and the protein in the waste marine products is adhered to the surfaces of the electrodes. This remarkably lowers the heavy metal deposition efficiency.
The deposition efficiency is lowered since the heavy metal, which is excessively deposited on the surfaces of the electrodes, is separated from the surfaces of the electrodes or the adhesive deposit including the heavy metal is adhered to the surfaces of the electrodes.
To address this problem, when the deposit including the heavy metal is excessively accumulated on the surfaces of the electrodes and the surfaces of electrodes become unclean, the surfaces of the electrodes are cleaned by rubbing the surfaces of the electrodes by brush, sponge, or the like to recover the deposition efficiency.
In the case of experimental equipment used in a laboratory, the surfaces of the electrodes can be rubbed by the brush and the sponge, but a large plant has dozens of electrodes with surface area of 1 m.sup.2 or more. Thus, the weight of the plant is in the metric ton level. In the case of the large plant, it is impossible to take out the electrodes from the electrolytic bath, clean the electrodes and then mount the electrodes in the electrolytic bath if the cleaning time and labor are taken into consideration.
Arsenic and phosphorus as well as the heavy metal are extracted into the acid solution, and thus, it is necessary to separate and collect arsenic as well as the heavy metal.